Studies of aerobic energy metabolism have identified the following electron carriers in the respiratory chain of Giardia lamblia and Entamoeba histolitica: pyridine nucleotides NAD(P)H, flavins, and non-heme Fe.S centers. Striking differences in the respiratory metabolism of the two organisms are shown by subcellular distribution and redox properties of the Fe.S centers. These carriers are localized in the particulate fraction of G. lamblia, and organism most intensely studied at present. Calmodulin, and a proton pump related to energy conservation has been found in the plasma membrane of this parasite. Mass cultivation of G. lamblia developed in our laboratory, facilitates studies of the novel findings. Collaborative studies show that kinetoplasts of Leishmania donovani promote the hydrolysis of ATP, and exhibit active respiration which responds differently to inhibitors of energy metabolism than their mammalian counterparts. Studies of mammalian mitochondria have shown that psychoactive drugs (reserpine) have unusual uncoupling properties of oxidative phosphorylation. Other studies of the effect of heme depletion in cultured murine erythroleukemic cells and rat hepatocytes disclose marked differences in their respiratory metabolism.